Ultrasonic imaging can be used to make accurate and precise measurements of structures of the eye, such as, for example, the cornea and lens. Such measurements provide ophthalmic surgeons with valuable information that they can use to guide various surgical procedures performed on the eye such as LASIK procedures or lens replacements.
Ultrasonic imaging of the cornea and lens presents a problem not generally encountered in other types of tissue. The corneal and lens surfaces are necessarily smooth and gently curved in order to perform their optical function of focusing light rays. Because these structures are smooth and regular, ultrasonic energy is reflected only in specific directions. In particular, an ultrasound pulse from a transducer will only be reflected directly back to that transducer when the pulse is reflected substantially at right angles from the corneal or lens surface. This kind of reflective property is call specular reflection. Because of the specular property of these surfaces, it will be appreciated that special care must be taken to align the transducer with the cornea or lens at each position from which an image segment is to be formed. Ultrasonic imaging of large portions of the cornea or lens can be accomplished by scanning the transducer along the component surface while continually adjusting the alignment of the transducer to provide a sequence of pulses that is always directed through the center of curvature of the specular component, thus ensuring normal reflection.
Corneal and lens imaging and measurement of dimensions require that the scanning motion of the transducer be smooth and precisely aligned. Departures of the transducer axis as small as 5 microns from the pulse's direction through the center of curvature can significantly degrade the resulting image. Mechanisms for performing the requisite scan alignment are described in U.S. Pat. No. 5,331,962, U.S. Pat. No. 6,491,637 and U.S. Pat. No. 6,887,203 which are incorporated herein by reference. Ultrasonic imaging may be used by ophthalmologists for quantitative analysis of laser refractive surgery, implantation of corneal and phakic lenses, implantation of intraocular lenses including accommodative lenses, and specialty procedures such as glaucoma and cataract treatment.
Except for on-axis measurements, images of eye components behind the iris and their dimensions cannot be determined by optical means. New procedures, such as implantation of accommodative lenses, may provide nearly perfect vision without spectacles or contact lenses. Implantation of accommodative lenses requires precision measurements of the natural lens and its suspensory ligaments for successful lens implantation. Such measurements include, for example, lens width, thickness, volume and location relative to the cornea. Ultrasonic imaging can be used to provide the required accurate images of the natural lens especially where its suspensory ligaments, known as zonules, attach to the ciliary body. The equatorial ends of the lens, the zonules and ciliary body are well off the optical axis, behind the iris and therefore not accessible to optical imaging.
Optical imaging devices can be used directly to image accessible portions of the interior of an eye. The speed of light in the cornea, aqueous humor, lens and vitreous humor varies from about 23% less than the speed of light in air to about 29% less than the speed of light in air in the lens. Furthermore the speed of light varies significantly throughout the lens depending on age and other factors. This makes optical measurements which depend on the transmission delays and hence actual speed of light difficult to transform from time delays to distance measurements.
Ultrasonic imaging requires a liquid medium to be interposed between the object being imaged and the transducer, which requires in turn that the eye, the transducer, and the path between them be at all times be immersed in a liquid medium. Many of the principal ultrasonic scanning mechanisms must be therefore submerged in water for long periods.
The speed of sound in the cornea, aqueous humor and lens is about 5 to 7% higher than the speed of sound in water. Furthermore, the speed of sound varies little throughout the lens even in the presence of cataracts. This makes acoustic measurements, which depend on the transmission delays of acoustic pulses, relatively easy to transform from time delays to distance measurements. So, in addition to being able to see the entire lens, acoustic imaging of the lens is less subject to errors in signal speed than optical imaging which is restricted to that portion of the lens visible through the pupil.
Normal ultrasonic imaging practice uses a single transducer for both sending ultrasound pulses to and receiving echoes from eye structures. That arrangement captures only those echoes that return directly to the transducer substantially along the transducer axis.
It is readily demonstrated that specular surfaces only return echoes along the axis of the incident pulse if the incident pulse is directed normal or perpendicular to the surface of the eye component of interest. This behavior has led to the development of ultrasound imaging devices that maintain their incident beam approximately perpendicular to the corneal or lens surface as the incident ultrasound pulses scan the surface. Such a device is described in U.S. patent application Ser. No. 12/347,674, entitled “Components for an Ultrasonic Arc Scanning Apparatus”, filed Dec. 31, 2008 and U.S. patent application Ser. No. 12/418,392 entitled “Procedures for an Ultrasonic Arc Scanning Apparatus” filed Apr. 3, 2009, both of which are incorporated herein by reference. With such a device, the incident pulse beam scans in a plane while directing its axis through a fixed center point. If that center point is at or near the center of curvature of the corneal or lens surface, the incident beam will remain approximately perpendicular to the surface throughout the scan, and ultrasound reflections will be returned to the transducer from all scanned parts of the surface.
One method of obtaining an image of the posterior surface of a natural or artificial implanted lens was disclosed in U.S. patent application Ser. No. 12/475,322 entitled “Compound Scanning Head for an Ultrasonic Scanning Apparatus”, filed May 29, 2009 which is incorporated herein by reference. This application discloses an ultrasonic arc scanning apparatus with an independently rotatable sector scan head mounted on the carriage of an arc scanning apparatus so as to form a compound scanning head. This invention presents an approach that allows the lens surfaces and cornea surfaces to be imaged at the same time.
There remains a need for more advanced ultrasonic scanning devices and methods that can rapidly produce a series of comprehensive images of the anterior segment of an eye, other than an arc scanner with a fixed radius of curvature such as described in, for example, U.S. Pat. No. 6,887,203.